The present disclosure generally relates to dual functioning electrodes adapted for anodic and cathodic use for a reverse current electrolytic chlorination apparatus such as may be desired for treating pool water, spas, and the like.
Electrolytic pool chlorinators have evolved to overcome the problems associated with chemical dosing of swimming pools, spas, and the like to prevent the accumulation growth of algae and bacteria therein. The electrolytic chlorinator generally includes two spaced apart electrodes including an anode for oxidation of chloride ions from, normally, sodium based chloride salts to chlorine, which subsequently hydrolyzes in solution to form hypochlorite; and a cathode for reduction of water to hydrogen. Water to be treated is dosed with the chloride salts and flows between the electrodes. The electrolytically generated chlorine and hypochlorite act as the active ingredients to oxidatively destroy bacteria and other harmful agents in the water.
One of the disadvantages associated with electrolytic disinfection is the cost of the electrolytic cell, as well as the cost of replacement electrodes, which can corrode, become fouled with scale and the like or otherwise become inactivated over time. These costs are primarily driven by the size of the electrodes, which are typically constructed from titanium coated with platinum or ruthenium. Electrodes having a surface area sufficient to generate adequate chlorine levels represent a significant portion of the cost of installing and maintaining an electrolytic disinfection system. In addition, electrolytic cell life is limited due to the current density through the cell over time.
In order to keep the electrodes clean and operating at maximum efficiency, the electrolytic current fed to the chlorinator can be configured with dual functional electrodes, wherein each electrode can dually function as the anode or cathode depending on whether the current flow is in the forward or reverse direction. This so called current reversal or reverse polarity operation exchanges the chemical reactions that occur on the respective electrodes and in doing so cleans the electrode surface. If mineral deposits are not removed from the chlorinator, the electrodes would soon cease to function because the deposits would cause the unit to reach a so-called “high voltage” cutoff, much like it does with current electrolytic cells that have single functioning electrodes.
One such dual functioning electrode is based on a coating of catalytic oxide mixture of ruthenium dioxide (RuO2) and titanium dioxide (TiO2) deposited onto a conductive substrate such as titanium. Based on its behavior as a continuous (uni-functional) anode, this particular mixed metal oxide is typically used at a mole ratio of about 40:60 to about 50:50 (RuO2:TiO2) formed on a titanium substrate. It is generally known that the operating lifetime of the coating for electrolytic applications depends to a large extent on the amount of the coating applied to the substrate. The total amount of ruthenium that is in a typical coating for electrolytic pool chlorinators is about 20 to about 30 g/m2 as Ru metal, application of which is generally provided by solvent coating multiple layers, typically about 20 to 30 coats. At ruthenium concentrations below 40 mole %, durability is known to significantly decrease when analyzing its capability as a continuous anode. For example, as discussed in an article entitled, “Optimization of an Anodic Electrocatalyst: RuO2/TiO2 on Titanium”, to Spasojevic et al. (J. Res. Inst. Catalysis, Hoklaido Univ. Vol. 31, Nos. 2/3, pp 77-94, 1983), when measuring the change in anode potential as a function of time for chlorine evolution at 3 kA/m2, 80° C. and constant brine concentration, it was observed that durability was at a maximum at 40 mol % RuO2 and decreased rapidly below 20%.
However, ruthenium is relatively expensive and efforts have been ongoing to reduce the amount of ruthenium used by use of less expensive metals. Because of this issue with durability when continuously functioning anodically without periodic reverse polarity, prior attempts to reduce the amount of ruthenium because of its expense have generally been directed to substitution of ruthenium with other metals, e.g., tin.
Accordingly, there remains a need for improved dual functioning electrodes that exhibit prolonged durability and use decreasing amounts of ruthenium.